The present invention relates to electroless gold plating, In particular, the present invention relates electroless gold plating at neutral pH.
The techniques for producing suitable metallic coating such as metallic pads for the electrical connection between electronic components are becoming increasingly demanding as the sophistication of electronic components Increases. For example, special printed circuit boards (PCBs) with non-conducted circuitry are often required for the fabrication of chip on board, ball grid array packages, multi-chip modules and chip scale packages. In many applications, gold pads are required for use in wire bonding or soldering techniques to interconnect the various electronic components.
Different methods exist in the art for producing gold pads for wire bonding or soldering. They are commonly divided into the electroless techniques and the electrolytic (Ely) techniques In both techniques, a barrier layer of metal is first deposited on the copper layer. The barrier layer is typically Ni2P for electroless techniques, and metallic nickel for electrolytic techniques. This is followed by deposition of the gold layer, Eleltroless gold techniques are in turn subdivided into the immersion gold methods and the reducing electroless gold (ES) methods.
Immersion gold methods involve a direct displacement reaction between gold ions and metallic nickel whereby, ionic gold becomes reduced while metallic nickel becomes oxidised. The technique typically involves a preliminary step of electroless plating of a nickel/phosphorus alloy (Ni2P) onto a substrate such as a PCB board or other electronic components using the following reaction:
4Ni2++8H2PO2xe2x88x92+2H2Oxe2x86x926H2PO3xe2x88x92+2Ni2P+H2(g)+6H+
The Ni2P plated substrate is then submerged in an immersion gold solution for the following redox reaction:
Ni2P+4[Au(CN)2]xe2x88x92xe2x86x924 Au+P+2[Ni(CN)4]2xe2x88x92
This two-step plating process is referred to as electroless nickel/immersion gold (ESN/IG) process. This reaction typically does not require a reducing agent, An ESN/IG plated pad (e.g. an input/output (I/O) pad on a PCB) is useful for soldering and aluminium wire bonding. However, the immersion reaction to deposit elemental gold onto the nickel plating causes oxidation and corrosion of the Ni2P deposit, resulting in the presence of a xe2x80x9cblack bandxe2x80x9d at the interface between the nickel layer and the gold layer. This gives poor results in gold wire bonding.
To improve gold wire bondability, JP 5222541 discloses a method in which a third electroless gold plating step (ESG) is performed after the above-described process i.e. ESN/IG/ESG. The general reaction for the ESG step is typically performed at acidic pH in the presence of a reducing agent:
2[Au(CN)2]xe2x88x92+H2PO2xe2x88x92+H2Oxe2x86x922Au+H2PO3xe2x88x92+4CNxe2x88x92+H2
The additional gold layer gives good bondability if the thickness of the ESG gold layer is 80 xcexcin or above, provided the thickness of the intermediate IG gold layer is carefully controlled to around 0.1 xcexcm, otherwise solderability is compromised. Furthermore, the undesirable xe2x80x9cblack bandxe2x80x9d corrosion problem remains unsolved.
To solve the xe2x80x9cblack bandxe2x80x9d corrosion problem, JP 98438 proposes the use of electroless palladium step before the immersion gold step (ESN/ESPd/IG). The pads produced by this method have good gold wire bondability, but poor solderability. Furthermore, the thickness required for the palladium layer is quite high, and the use of two precious metals result in high costs.
The only method currently known in the art to produce metal pads that have both good solderability and good gold wire bondability is electrolytic soft gold plating. This method, however, require that the plating surfaces be electrically connected for electrolysis, thus posing a severe limitation on board design. Furthermore, the thickness of the gold deposit lacks uniformity, being directly dependent on the distribution of current density.
It is therefore an object of the present invention to provide a method of producing a metallic layer that is solves the aforementioned prior art problems.
Accordingly, the present invention provided, in one aspect, an electroless gold plating method that minimises xe2x80x9cblack bandxe2x80x9d corrosion problem in the art. The electroless gold plating reaction according to present invention (referred to hereinafter as EG-1) may be performed on a typical Ni2P plated layer on a substrate. The EG-1 electroless gold plating solution is provided at neutral pH in the presence of a reducing agent to prevent corrosion of the Ni2P plating on the substrate, and a complexing agent to maintain the gold ions in solution. The solution is further provided with an accelerator to increase the rate of reduction, and to allow a gold layer of the desired thickness to be plated under manufacturing conditions. The product produced therefrom contains a substrate having a first Ni2P layer plated by conventional electroless Nickel plating, and a second gold layer plated above the Ni2P layer in the absence of a black band of phosphorus when traverse cross-section of the product is observed under a scanning electron microscope of up to 10,000xc3x97 magnification and using the technique of Energy Dispersive x-ray (EDX).
In a further aspect of the present invention, a second electroless gold plating step (hereinafter referred to as EG-2) is provided after EG-1 to further increase the thickness and purity of the gold layer. The plating solution for EG-2 may be the same the EG-1, or the gold ion concentration is preferably higher. The product produced thereof is also a substrate with a gold layer plated on a Ni2P layer in the absence of a phosphorus black band.
The thickness of the gold layer plated according to the present method need only be 10-20 xcexcin to achieve both good solderability and good gold wire bondability. This is thinner than the conventional soft gold pads, which typically require 25 xcexcin thickness or higher.
The gold pad produced according to the present method possesses both excellent bondability and solderability.
The following detailed description describes the preferred embodiment for implementing the underlying principles of the present invention. One skilled in the art should understand, however, that the following description is meant to be illustrative of the present invention, and should not be construed as limiting the principles discussed herein. In the following discussion, and in the claims the terms xe2x80x9cincludingxe2x80x9d, xe2x80x9chavingxe2x80x9d and xe2x80x9ccomprisingxe2x80x9d are used in an open-ended fashion, and thus should be interpreted to mean xe2x80x9cincluding but not limited to . . . xe2x80x9d. As used herein, electronic components include electronic devices, integrated circuit devices, and printed circuit boards. Substrate include dielectric material, epoxy and other material commonly used for packaging electronic circuitry and devices.
The electroless gold plating method according to the present invention is preferably used on a conventional electroless Ni2P deposition onto a substrate surface. The Ni2P acts as a barrier layer to protect the copper traces in the substrate. The substrate is typically made from organic material, e.g. epoxy or dielectric material for PCB fabrication. The Ni2P deposition is performed by first cleaning the substrate by acid, followed by microetching to expose the surfaces to be plated. After treatment in an acid bath, the surface to be plated is then activated before electroless nickel plating occurs. The general reaction is as follows:
4Ni2++8H2PO2xe2x88x92+2H2Oxe2x86x926H2PO3xe2x88x92+2Ni2P+H2(g)+6H+
This is followed by electroless gold plating (EG-1) in a solution containing KAu(CN)2, citrate salt as the complexing agent, sodium hypophosphite as the reducing agent, and thallium salt as the accelerator. The solution is maintain at a pH of between 6.5 to 7.0 and the reaction allowed to occur for 1-12 minutes, For a thicker layer of gold, a second electroless gold plating step (EG-2) may be performed by submerging in a solution containing the same components as the EG-1 solution, or a solution with a higher KAu(CN)2 concentration. The citrate salt may be any salt of citrate including but not limited to sodium, potassium or ammonium salt. Citrate also functions as a buffer to maintain the solution at neutral or near-neutral pH. The Thallium salt may be any salt of Thallium, including but not limited to chloride, bromide or acetate salt. The following examples describes the detailed process of the present invention: